But all the major characters from FC have been announced. Everyone who hasn't was either a small supporting or tiny thankless role. The FC will consist of the "new younger characters" Singer mentioned.

Exactly. Four returning major characters from FC does not equal four characters in the 1970s.

Also, it's possible that the OT characters announced so far would be the only characters we see in the future, period, if it takes place in some devastated post-apocalyptic world. Whereas I imagine there will be quite a few background characters in the 70s along the lines of Oliver Platt's character in FC, and also some mass scenes according to that casting notice from some months ago.